[gnutls-devel] libtasn1 | coding.c: fix undefined behavior with pointer arithmetics (!63)

[Development of GNU's TLS library]

Alexander Us created a merge request: https://gitlab.com/gnutls/libtasn1/-/merge_requests/63

Project:Branches: alexander-us/libtasn1:clang_10_ptr_arith_ub to gnutls/libtasn1:master
Author:    Alexander Us



`asn1_der_coding` contained unchecked pointer arithmetics.

As source code shows, `ider` can be `NULL` pointer. C standard states that arithmetic using `NULL` pointers gives undefined behavior (C99, 6.5.6, clause 8). LLVM (beginning from version 10) started to optimize pointer arithmetic in comparisons with `NULL` with assumptions that addition of `NULL` and non-zero value will produce undefined behavior (https://reviews.llvm.org/D66608). This means that code like `ptr + x == NULL` will be turned to `ptr == NULL` because if ptr is `NULL` then `NULL + x` will be UB. Short summary of what happened in `asn1_der_coding` is in this code: https://godbolt.org/z/DAo0Bt.

Without my patch clang UBSan reports problems in **copynode** and **Test_tree** tests and these tests fail with `-O2` flag (`CC=clang-10 CFLAGS="-O2 -fsanitize=undefined"`):

```
../../libtasn1/lib/coding.c:1207:56: runtime error: applying non-zero offset 7 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../libtasn1/lib/coding.c:1207:56 in
../../libtasn1/lib/coding.c:1225:54: runtime error: applying non-zero offset 287 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../libtasn1/lib/coding.c:1225:54 in
LIBTASN1 ERROR: VALUE_NOT_VALID
Cannot copy node
FAIL copynode (exit status: 1)
```

## Checklist
 * [ ] Code modified for feature
 * [ ] Test suite updated with functionality tests
 * [ ] Test suite updated with negative tests
 * [x] Documentation updated

## Reviewer's checklist:
 * [ ] There is a test suite reasonably covering new functionality or modifications
 * [ ] Function naming, parameters, return values, types, etc., are consistent with other code
 * [ ] This feature/change has adequate documentation added
 * [ ] No obvious mistakes in the code

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